Especially high safety-technological demands must be made on controllers where control malfunctions can cause injury to persons. For the enhancement of the safety of electronically controlled production machines numerous possibilities are known (Dipl. Ing. W. Wienandts "Sicherheitstechnische Anforderungen und electronische Steuerungen" published in "Die BG", March 1980, pp. 220-224). FIG. 2 of the cited reference shows a press controller with two identically constructed control channels whose individual stages are monitored by a safety channel by means of comparison. The safety channel in turn contains a self-check feature. In a system of this type, system errors are possible, e.g. design errors, which are not recognized. It does thereby not offer the necessary safety.
FIG. 6 of the cited publication shows the systemic structure of an electronic controller for a moving staircase. It has one control channel and two safety channels, one of which is constructed with hard-wired switching logic and the other with microcomputer technology. Owing to different technologies utilized in processing the signals, the probability of systematic errors affecting the two control channels simultaneously and in the same manner is considerably reduced. Nevertheless, the probability of simultaneously occurring malfunctions in the first and second safety channels is unacceptably high, since on account of the great number of integrated components in the safety channels with microcomputer technology there exists a high probability of malfunction for this channel. Self-check facilities in this safety channel can reduce the effects of frequent errors. A self-check with a high rate of malfunction recognition, however, requires a time which, depending on circumstances, can last several seconds. This monitoring time may undesirably prolong the cycle time in devices operating cyclically and with periodic monitoring of this device and may thus decrease the availability and utilization of the device.